Fix handling prepending field if target blank or prepend is multival
For target_field
=';foo' and prepended_field
='q', it was returning
'q: ;q: foo'. Now it returns ';q: foo'
For target_field
=';foo' and prepended_field
='q;t', it was returning
'q: ;t: foo'. Now it returns ';t: foo'
For target_field
='foo;bar;baz' and prepended_field
='q;t', it was returning
'q: foo;t: bar;t:baz'. Now it returns 'q: foo;t: bar;baz'